Achondroplasia

Definition

Achondroplasia is a genetic disorder that causes short stature called dwarfism and a large head, also known as macrocephaly. It is a disorder in which bone and cartilage do not grow normally. It is the most common cause of dwarfism.

This condition leads to people reaching a full-grown height of around 4-4½ feet. The greatest shortening occurs in the bone between the shoulder and the elbow and the bone between the hip and the knee. There may also be underdevelopment of the face.

Causes

Achondroplasia is a genetic disorder. It is caused by mutations in the FGFR3 gene. This gene prevents the growth of cartilage at the growth plate. FGFR3 encodes a protein called Fibroblast Growth Factor Receptor 3. This protein is the site of action of a major growth factor responsible for lengthening bones. When this growth factor cannot act properly due to the absence of its receptor, the growth of bones, at the growth plate's cartilage, is slowed. This leads to shorter bones, abnormally shaped bones, and shorter stature.

The gene for achondroplasia can be passed from one generation to the next. If a parent has the disorder, there is a 50% chance of passing the gene for achondroplasia to children. In most cases of achondroplasia, it more commonly is the result of a sudden genetic defect that occurs in the developing embryo.

Risk Factors

Factors that may increase the chance of achondroplasia include:

Children of a parent with achondroplasia

Children of normal-sized parents who carry a mutated FGFR3 gene

Advanced paternal age causing spontaneous mutations

Symptoms

Symptoms of achondroplasia include:

Short stature, a normal trunk, and shortened limbs, which are noticeable at birth

An adult height around 4-4½ feet

A head that is large and a forehead that is prominent

Portions of the face can be underdeveloped

At birth, the legs appear straight, but as a child begins to walk, he or she develops a knock-knee or bowed-leg deformity

The hands and the feet appear large, but the fingers and toes are short and stubby

Straightening of the arm at the elbow may be limited, but usually does not keep someone from doing any specific activities

Children may develop an excessive curve of the lower back and a waddling walking pattern

Dental problems

Other common symptoms include:

Weight control problems

Dental problems from overcrowding of teeth

Neurologic and respiratory problems

Fatigue, pain, and numbness in the:

Lower back

Spine

Diagnosis

The doctor will ask about your symptoms and medical history. A physical exam will be done. The strength of your extremities and your bladder control will be evaluated. Weakness and loss of bladder control are both signs of spinal stenosis, which is a narrowing of the spinal canal. It is important to follow the doctor's advice to make sure that
spinal stenosis
does not develop without being recognized. This generally occurs between the ages of 20-40 years.

Treatment

At this time, there is no cure for achondroplasia.
Scientists are exploring ways to create alternate growth factors which can bypass the missing receptor and lead to normal bone growth. They may offer the possibility of enhanced stature to future families who have children with achondroplasia.

Medication

Treatment with human growth hormone has been used for over a decade. It effectively increases bone growth rate, at least in the first year of life. There have been few studies looking at whether children treated with growth hormone achieve greater adult height.

Surgery

Laminectomy—Opens the spinal canal to relieve pressure on the compressed spinal cord from
spinal stenosis.

Osteotomy—Bones of the leg are cut and allowed to heal in the correct anatomical position. This procedure is for those with severe knock-knee or bowed legs.

Osteotomy has primarily been used to correct deformities. In recent years, bone lengthening procedures have been used for many short-statured children, including those with achondroplasia. The procedures are lengthy, traumatic, and demanding for both children and their families. Complications, sometimes serious, are common. One center has reported an average leg length gain of about 7 inches and an average increase in arm length of about 4 inches for achondroplastic individuals who undergo surgery. The combination of growth hormone therapy followed by lengthening surgery may provide benefit in achieving near-normal stature and proportions.

Prevention

There are no steps to prevent a genetic disorder. Genetic counseling can be used to discuss the chances that your child will have achondroplasia. You may consider this counseling if you are planning on having a child and have a family history of genetic conditions.

Revision Information

This content is reviewed regularly and is updated when new and relevant evidence is made available. This information is neither intended nor implied to be a substitute for professional medical advice. Always seek the advice of your physician or other qualified health provider prior to starting any new treatment or with questions regarding a medical condition.